Cells (Chapter 3)

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Cells

the basic structural and functional units of all living organisms, allowing for various biological processes.

200

the amount of cells a human body has with diverse shapes, size, and functions.

Cell Membrane

the outer protective layer of a cell, which encloses and supports all its componentsl

Homeostasis

The process by which living organisms regulate their internal environment to maintain a stable, constant condition.

Feedback System

This monitors the output of a process and uses that information to adjust the input. It’s a loop that helps maintain stability or amplify change, depending on the type.

Negative Feedback

Reduces the effect of a stimulus to maintain balance. (Shuts off original stimulus)

• Most homeostasis control mechanisms.

Positive Feedback

Rises the effect of a stimulus, often until a specific outcome is reached. (Increases the original stimulus)

• Not directly used for homeostasis, associated with injury and diseases.

Stimulus, Receptor, Input, Output, Response

Ordered Variables in a Homeostatic Control System

Stimulus

A change occurs in the internal or external environment (e.g., temperature drops).

• Produces charge

Receptor

Specialized cells detect the change and send signals.

Input

The signal sent from the receptor to the control center, often via nerves or hormones.

Output

The signal sent from the control center to the effector to initiate a corrective action.

Response

The action taken by the effector that restores balance.

Cytoskeleton

A complex, dynamic network of protein filaments and tubules in the cytoplasm of many living cells, extending from the nucleus to the cell membrane. It is made up of three main types of protein filaments.

• Microfilaments

• Intermediate Filaments

• Microtubules

Microfilaments

Structure: Thin, flexible strands (~7 nm diameter) made of actin.

• Function:

  • Support cell shape.

  • Enable cell movement (e.g., amoeboid motion).

  • Drive muscle contraction (with myosin).

  • Help in cytokinesis (cell division).

Intermediate

Structure: Rope-like fibers (~10 nm diameter), made of various proteins like keratin or vimentin.

• Function:

  • Provide mechanical strength.

  • Anchor organelles.

  • Maintain cell integrity under stress.

Microtubules

Structure: Hollow tubes (~25 nm diameter) made of tubulin (α and β subunits).

• Function:

  • Form the mitotic spindle during cell division.

  • Act as tracks for motor proteins (e.g., kinesin, dynein) to transport cargo.

  • Shape the cell and position organelles.

  • Build cilia and flagella for movement.

Cell Extension

These are specialized projections from the surface of cells that enhance their interaction with the environment. These structures can help with movement, absorption, or sensing, depending on their type. There are three main kinds of extension.

• Cilia

• Flagella

• Microvilli

Cilia

Structure: Hair-like projections made of microtubules.

• Many Cell

Flagella

Structure: Long, whip-like tail made of microtubules.

• 1 per cell

Microvilli

Structure: Tiny, finger-like projections supported by actin filament.

• Mainly found in intestine, Increases surface area

Cell division

The biological process by which a parent cell splits into two or more daughter cells. (Multiplies cells) It’s essential for growth, repair, reproduction, and maintaining healthy tissues. There are two main types:

• Mitosis

• Meiosis

Mitosis

Purpose: Produces two genetically identical cells.

• Occurs in: Somatic (non-reproductive) cells.

• Phases:

  • Prophase: Chromosomes condense, spindle forms.

  • Metaphase: Chromosomes align at the center.

  • Anaphase: Sister chromatids separate.

  • Telophase: Nuclear membranes reform.

  • Cytokinesis: Cytoplasm divides, forming two cells.

• Result: Two diploid cells (same chromosome number as parent).

Meiosis

Purpose: Produces gametes (sperm and egg) for sexual reproduction.

• Occurs in: Germ cells.

• Phases: Two rounds—Meiosis I and Meiosis II.

• Result: Four haploid cells (half the chromosome number).

• Key Feature: Genetic variation through crossing over and independent assortment.

Binary Fission

Simpler process used by bacteria.

• DNA replicates, cell elongates, and splits into two.

Plasma Membrane

The outer boundary of the cell.

• Function:

  Controls what enters and exits the cell (selective permeability).

  Maintains the cell’s internal environment.

  Communicates with other cells via receptors.

• Structure: A flexible phospholipid bilayer with embedded proteins and cholesterol for stability.

Cytoplasm

The jelly-like substance inside the cell, excluding the nucleus.

• Function:

  Suspends organelles.

  Site of many metabolic reactions.

  Helps transport materials within the cell.

• Structure: Includes cytosol (fluid) and organelles like mitochondria, ribosomes, and the endoplasmic reticulum.

Nucleus

The control center of the cell.

• Function:

  Stores DNA, the genetic blueprint.

  Directs cell activities like growth, metabolism, and reproduction.

• Structure: Surrounded by a nuclear envelope. Contains chromatin and the nucleolus (makes ribosomes).

Passive Transport

No energy required

Substances move down their concentration gradient (from high to low)

Simple Diffusion

(TYPE OF PASSIVE TRANSPORT)

Small, nonpolar molecules (like O₂, CO₂) pass directly through the lipid bilayer.

Carrier-Mediated Facilitated Diffusion

(TYPE OF PASSIVE TRANSPORT)

Larger polar molecules (like glucose) move via protein channels or carriers.

Channel-Mediated Facilitated Diffusion

(TYPE OF PASSIVE TRANSPORT)

Small polar molecules move via protein channels or carriers.

Osmosis

(TYPE OF PASSIVE TRANSPORT)

Diffusion of water across a selectively permeable membrane.

Active Transport

Requires energy—usually ATP

Substances move against their concentration gradient (from low to high).

Primary Active Transport

(TYPE OF ACTIVE TRANSPORT)

Uses ATP directly (e.g., sodium-potassium pump).

Secondary Active Transport

(TYPE OF ACTIVE TRANSPORT)

Uses energy from another gradient (e.g., glucose co-transport with sodium).

Bulk Transport

For large molecules or particles

Endocytosis

Cell encloses/engulfs material into vesicles.

Phagocytosis

(TYPE OF ENDOCYTOSIS)

Cell eating

Pinocytosis

(TYPE OF ENDOCYTOSIS)

Cell drinking

Receptor-mediated endocytosis

(TYPE OF ENDOCYTOSIS)

Main cellular mechanism.

Exocytosis

Vesicles fuse with membrane to release contents outside the cell.

Lysosomes

The Cell’s Digestive System

• Function: Break down waste materials, damaged organelles, and foreign invaders.

Phagocytes

The Cellular Bodyguards.

• Function: Engulf harmful particles, bacteria, and dead cells via phagocytosis.

Peroxisomes

The Detox Units

• Function: Break down fatty acids and neutralize toxic substances like hydrogen peroxide.

Tonicity

Ability of a solution to change size and shape by altering the amount of water they contain.

Isotonic

Same solute and water concentration. Cell stays the same

Hypotonic

Fewer solute. Cell swells, may burst

Hypertonic

More solute. Cell shrinks

Central Dogma of Biology

Conversion of Information.

• DNA - RNA - Protein

  • Replication - Transcription - Translation

Cytokinesis

Resting stage depending on cell.